This invention relates to a method for producing mica scales by size reduction of mica flakes and an apparatus used therefor. More particularly, this invention relates to a method for reducing in size of mica flakes continuously and economically effectively to mica scales having the desired thickness and particle size distribution and an apparatus used therefor.
Heretofore, various methods for producing mica scales have been proposed, for example, in Japan Pat. application Kokoku (Post-Exam. Publn.) No. 15351/60, U.S. Pat. Nos. 2,405,576 and 3,608,835, etc.
The method disclosed in Japan Pat. application Kokoku (Post-Exam. Publn.) No. 15351/60 is a method for disintegrating calcined mica which comprises heating mica flakes so as to partially remove the water of crystallization but not the whole at 700.degree.-900.degree. C. in order to expand the volume between the layers of a mica flake (hereinafter referred to as "volume expansion" or "volume expanded"), dipping the volume expanded mica flakes in water or an aqueous solution of an acid or an alkali (hereinafter referred to as "an aqueous solution") as they are at high temperatures or after cooling, and stirring the aqueous solution in order to disintegrate the mica flakes to mica scales having 1.mu. or less in thickness by the mechanical force of the aqueous solution. The apparatus used for this method is shown in FIG. 1. According to this method, the volume expanded mica flakes 6 fed together with water from a raw material feed section 2 into a disintegration tank 1 are disintegrated by a stirrer 7 for a long period of time, and mica scales 5 having a smaller size than the opening in a perforated plate 4 at the overflow section can be obtained from a overflow pipe 3 in the form of mica scale suspension. But this method has a fatal defect in that it is very difficult to conduct a stable continuous operation without improving the apparatus for preventing closing of the perforated plate 4, since the volume expanded mica flakes or larger mica scales easily close the perforated plate 4 at the overflow section. Therefore, this method has many defects in that mass productivity is poor, much more steps are necessary and economy of the process is very poor. In addition, according to this method, it is possible to disintegrate volume expanded mica flakes 6, but it is impossible to disintegrate non-volume expanded mica flakes to mica scales. Thus, there are many problems in this method in that it is necessary to calcine mica flakes for volume expansion, thickness and particle size distribution of the resulting mica scales are changed depending on the degree of calcining, i.e., the degree of volume expansion, in case of some kinds of mica flakes, the components are oxidized during the calcining resulting in remarkable decrease in mechanical strengths, and the like.
The method disclosed in U.S. Pat. No. 2,405,576 comprises using an "8" shaped disintegration tank having an upper chamber and a lower chamber connected by an intermediate section, charging mica flakes in the lower chamber, introducing high-speed jets of water from nozzles at the intermediate section to the lower chamber to disintegrate the mica flakes, and taking out the resulting disintegrated mica scales from the upper chamber. This method has various defects due to the structure of the disintegration tank in that once produced thinly delaminated mica scales having a large size are further disintegrated to smaller particles having a small ratio of diameter to thickness during the residence in the lower chamber and the upper chamber by beat against one another or against mica flakes resulting in decrease in mechanical and electrical properties; since the disintegration tank has no classification effect, large mica flakes are taken out of the tank together with the disintegrated mica scales, which gives various problems in producing mica paper; and the like.
U.S. Pat. No. 3,608,835 discloses a method for producing mica scales by using a disintegration tank of an inverted truncated cone shape having at the bottom agitating blades which coarsely disintegrate mica flakes by revolution, and having at the side wall an ultrasonic generator which disintegrates the coarsely disintegrated mica flakes to mica scales by ultrasonic energy. In this method, desirably disintegrated mica scales are taken out of the disintegration tank together with overflowing water and large mica flakes are disintegrated again in the tank. But since the disintegration tank has no sufficient classification effect, large mica flakes are often taken out of the tank together with mica scales by overflow; this is a fatal defect of this method.
Mica scales of several microns or less in thickness obtained by disintegrating mica flakes by these methods mentioned above are dispersed in water and mica paper is made from said pulp-like mica scale suspension or slurry by using a modified paper machine followed by dehydration and drying. Since mica paper is made by reconstituting mica scales in foil or sheet, it is also named as reconstituted mica sheet.
According to the conventional methods as mentioned above, large or thick insufficiently disintegrated mica scales are inevitably present in desirably thinly disintegrated mica scales, so that these mica scales are not suitable for producing mica paper. Thus, it is necessary to classify the slurry discharged from a disintegration tank in another step. Further, since disintegration tanks having insufficient classification effect are used in the conventional methods as mentioned above, thinly and largely disintegrated mica scales are ground to small particles during disintegration in the tank; this is not a desirable disintegration method.
In addition, the conventional methods as mentioned above have also a defect in that these methods cannot be applied to all mica flakes of non-calcined phlogopite, non-calcined muscovite and calcined muscovite.